1. Field of the Invention
The present invention relates to a duplexer used in, for example, a portable terminal device and the like.
2. Description of the Related Art
In a device having a two-way radio communication function such as a mobile phone, and a radio communication system using this type of device as a communication terminal, in order to transmit/receive signals using one common antenna provided in the communication terminal, a transmission signal and a reception signal are separated in a duplexer (elastic wave duplexer) including a transmitting side filter and a receiving side filter by providing a difference between a frequency of transmission signal and a frequency of reception signal, and reducing a level of signal flowing between the transmitting side filter and the receiving side filter to realize a preferable isolation characteristic, as shown in FIG. 20.
Concretely, the duplexer includes, as shown in FIG. 21(a), for instance, a low band (transmitting) side filter 11 for transmitting a transmission signal from a transmission input port 15 to a not-shown antenna port, and a high band (receiving) side filter 12 for receiving a reception signal at a reception output port 16 via the antenna port. The respective filters 11, 12 are formed of ladder-type filters each formed of SAW (Surface Acoustic Wave) resonators 200 being series arms and SAW resonators 201 being parallel arms, and are formed on mutually different piezoelectric substrates 10a, 10b, for instance. Reference numerals 17, 18 in FIG. 21(a) denote ground electrodes. Note that in the duplexer, a not-shown phase shifter is provided to prevent the reception signal from entering the transmitting side filter 11.
When the transmission/reception of signals is conducted in the duplexer, in each of the filters 11, 12, capacitive couplings C1 to C3 and C4 to C9 are respectively formed so as to stride over the plurality of SAW resonators 200, 201 via an upper area or an internal area of the piezoelectric substrate 10, for instance, as shown in FIG. 21(b). Accordingly, due to the capacitive couplings C1 to C3 formed in the transmitting side filter 11, for instance, the isolation characteristic in a pass frequency band of the receiving side filter 12 is deteriorated, as also shown in later-described FIG. 4.
Further, as shown in FIG. 22(a), for instance, when the filters 11, 12 are formed on one piezoelectric substrate 10 to miniaturize the duplexer, the filters 11, 12 are disposed close to each other. Accordingly, as shown in FIG. 22(b), an influence of capacitive couplings C10 to C21 generated between the filters 11 and 12 becomes large in addition to that of the aforementioned capacitive couplings C1 to C9, which further deteriorates the isolation characteristic in the pass frequency band of the receiving side filter 12. Note that in FIG. 22(b), the illustration of parallel arms (SAW resonators) 201 is omitted.
In addition, the conventional duplexer also has problems as follows. Specifically, a frequency signal with double the pass band of the receiving side filter 12 (double frequency signal) becomes a main cause of increasing a harmonic signal level in a device on a rear stage side of the receiving side filter 12, so that there is a request to increase an attenuation in the double frequency. For this reason, there is required a function for adjusting a position of maximum attenuation 3 that is generated on a high band side of the pass band of the receiving side filter 12, which is, for instance, in the vicinity of 1500 MHz, as shown in FIG. 23. For example, as a method of shifting the position of the maximum attenuation 3 to the low band side, there is known a method of adjusting a length and the like of routed electrodes and bonding wires on a package side connected to ground electrodes 17, 18, or electrodes connected to these ground electrodes 17, 18 at an external of the device, for example, to thereby increase inductance components of these electrodes. However, in the above method, there is a need to fabricate a package in accordance with the device connected to the rear stage side of the duplexer, for example, so that time and cost have to be spent for fabricating the packages, and further, when the ground electrodes 17, 18 are grounded using bumps, it is difficult to increase inductance components of the bumps. Further, as a method of shifting the position of the maximum attenuation 3 to the low band side, there is also known a method of increasing capacitances of the parallel arms 201 and decreasing capacitances of the series arms 200, but, with these methods, an insertion loss in the pass frequency band of the filters 11, 12 becomes large.
Although Patent Document 1 discloses a technique for appropriately setting a bridging capacitor C when improving an attenuation characteristic in a surface wave filter, no description has been made regarding the aforementioned problems. Further, although Patent Document 2 discloses a structure for improving an isolation characteristic in a duplexer, no study has been made on the maximum attenuation 3.
[Patent Document 1] International Publication WO 2005/101657 (paragraphs 0010, 0012, and FIG. 10)
[Patent Document 2] International Publication WO 2006/016544 (FIG. 14)